The technical field of the present invention is ophthalmic surgery and, in particular, instruments for removal of corneal tissue.
A microkeratome is a medical instrument used for resecting a thin layer of corneal tissue from the surface of the eye. In ophthalmic surgery, microkeratomes are used for various purposes. These purposes include the removal of abnormal growths in the cornea, preparation of damaged eyes for corneal transplants, preparation of eyes for other surgical procedures and direct surgical corrections of refractive disorders.
Considerable interest has been recently generated in a variety of techniques for reshaping the cornea for refractive vision correction. These techniques are based on the observation that most of an eye""s refractive power is contributed by the corneal curvature itself (with the remaining refractive power being provided by the lens of the eye located inside the ocular globe). For people suffering from near-sightedness (myopia), it has been recognized that a slight flattening of the corneal curvature can correct this condition if properly applied. Conversely, correction of far-sightedness (hyperopia) requires a steepening of the corneal curvature. Correction of astigmatism typically requires more complex reprofiling.
It has been suggested on a number of occasions that it is possible to correct refractive errors by mechanical sculpting of the cornea into an ideal shape and curvature. However, until very recently, there have been no tools suitable for this purpose. The anterior surface of the cornea is covered with a thin layer of epithelial tissue followed by a membrane-like structure known as Bowman""s layer. Typically, Bowman""s layer is about 30 micrometers thick, although it may vary from as little as 10 micrometers to over 50 micrometers in thickness.
Below Bowman""s layer lies the stroma proper of the cornea. This stromal tissue is approximately 450 micrometers in thickness, although it also varies from individual to individual. Stromal tissue is composed of a highly organized matrix of acellular collagen. The Bowman""s membrane that lies above it is less regular and denser.
Efforts at mechanical sculpting of the cornea have been largely unsuccessful to date because even the sharpest metal (or even diamond) blades are incapable of producing precise ablations of corneal tissue with the necessary accuracy. The irregularity of Bowman""s layer is a further complicating factor that has stymied mechanical attempts at wide-area sculpting of the anterior surface of the cornea.
In an alternative surgical procedure, an anterior segment of the cornea is removed (or partially severed and displaced) by a microkeratome so that the stromal bed can be mechanically sculpted. Because Bowman""s layer is removed or displaced intact in such procedures, mechanical instruments (e.g., specially designed microkeratomes and the like) have had moderate success in resculpting the stroma proper. After the stromal bed has been surgically reshaped, the anterior lenticule is replaced. Again, this procedure has the advantage of avoiding mechanically shaving Bowman""s layer, albeit at the expense of a deeper penetration into the stroma.
Recently, a new procedure, known as xe2x80x9claser vision correctionxe2x80x9d has become available to ophthalmologists to perform corneal surgery. Laser vision correction employs high energy pulses of ultraviolet radiation, typically from excimer lasers, to ablate thin layers of corneal tissue by a process known as xe2x80x9cphotodecomposition.xe2x80x9d This laser vision correction process relies upon the ability of such laser radiation to remove extremely thin layers of corneal tissue within an exposed area without thermal damage to adjacent tissue. In one type of procedure known as photorefractive keratectomy (PRK), the laser beam is either repeatedly scanned across the cornea or otherwise controlled to expose the cornea to a beam of different shape or size over time so as to effect a cumulative reprofiling of the corneal surface.
In a particular class of PRK procedures known as Laser Assisted In Situ Keratoplasty (LASIK), a microkeratome is used to remove (or hingedly displace) an anterior lamina of the cornea (in much the same way as in the procedures that involve mechanical sculpting of the stroma) while a laser is used to selectively ablate stromal tissue. Again, like mechanical sculpting procedures, the anterior lamina is replaced following the procedure with Bowman""s membrane intact. This LASIK procedure is also very promising but likewise requires precision in the removal of the lamina.
The microkeratome typically includes an eye ring assembly for placement on the ocular globe such that a portion of the cornea is secured. A cutting blade is then carried along a cutting path defined, at least in part, by the guide ring or other elements connected to the guide ring.
In ophthalmic surgery, the dimensions of the resection must be very precise and predictable. Precision can depend on several factors, including the pressure exerted by the surgeon""s hands on the instrument and on the patient""s eye, and the speed at which the blade is pushed to make the resection. Even when the movement of the blade is automated, there are factors that affect the precision of the cut.
Various techniques have been proposed for improving precision. For example, U.S. Pat. No. 5,980,543 to Carriazo et al. describes a microkeratome having a float arm compressing the surface of a cornea in front of the blade prior to cutting. The float arm is connected to the cutting head to, at least partially, compress the cornea ahead of the blade so as to set the desired thickness of the corneal resection. Carrizo""s blade assembly is exemplary of the prior art technique for coupling a blade to the body of the keratome.
The problem of controlled movement across the guide ring is addressed in U.S. Pat. No. 5,133,726 to Ruiz et al., which discloses a microkeratome with a mechanical drive assembly that provides a uniform mechanical motion. The blade assembly disclosed in Ruiz is introduced into the cavity in such a way that the blade is held parallel to the underside of the upper body of the microkeratome.
U.S. Pat. No. 5,817,115 to Nigam discloses an alternative instrument for making corneal incisions wherein a vacuum holds the cornea in place while an incision is made by a blade sliding through the instrument. The blade is mated to and driven by a plunger which actuates within a recess in the instrument. When the plunger is forced into the instrument, a spring is put in compression thus tending to push the plunger in the direction opposite the plunger""s movement. This arrangement purportedly slices the cornea in a single continuous motion.
There exists a need for better microkeratomes, generally, to facilitate both mechanical and laser vision correction procedures. A better, more accurate keratome, would allow ophthalmic surgeons to perform therapeutic keratectomies (removing small regions of corneal tissue which exhibit abnormal growths or ulcers), resections of anterior corneal segments (as a first step in keratomileusis, stromal sculpting procedures, LASIK procedures and the like) and a variety of other surgical operations on the cornea.
It has been discovered that unintended movements of the blade assembly occur during use of many microkeratomes and such movements (typically involving a xe2x80x9cwobblingxe2x80x9d motion of the blade) degrade both the precision and the reproducibility of results. The source of this errant motion can be traced in many instances to the fit of a disposable blade assembly within the microkeratome body. Many commercially available microkeratomes operate with a disposable blade that is intended for use on only one patient. Following each procedure, the blade must be replaced. A conventional design provides for the blade to be bonded to a post that permits easy handling by the clinician. This blade and post assembly is commonly referred to as a xe2x80x9cblade assembly.xe2x80x9d A number of commercially available microkeratomes are designed with a recess into which the blade assembly is placed. Although the assembly is typically designed to fit within the recess, a small amount of clearance is necessary to facilitate insertion and removal of the blade assembly. As a consequence of this clearance, the blade itself is only loosely secured during operation and largely held in place by the pressure exerted against the blade by the corneal tissue during resection.
Accordingly, methods and devices are disclosed to inhibit blade wobbling. In one aspect of the invention, frictional fittings are employed to secure the blade (or blade assembly) within the microkeratome. Various engagement elements are disclosed for immobilizing the post of a blade assembly upon insection into a microkeratome housing. The engagement elements preferably have spring-like characteristics. By biasing the blade into a secure position, wobbling during usage is substantially lessened if not eliminated.
In one aspect of the invention, a microkeratome is provided for creating lamellar sections from a biological tissue containing a blade-post assembly and at least one engagement element that improve precision and predictability by preventing undesired blade wobbling.
In another aspect of the invention, a modified blade assembly is disclosed having a post that is mated to a surgical blade. In one embodiment of the invention, the post can have one or more protrusions that engage at least one side of a recess in the microkeratome thereby creating a frictional fit sufficient to prevent movement of the blade while cutting. For example, the post can bias a portion of the blade assembly against an inner surface of the recess. In another embodiment of the invention, the recess can include certain biasing elements that engage the post to prevent blade wobbling. Further, the post can contain a coupling that permits transverse oscillation of the blade while the engagement element prevents wobbling motions.